1. Technical Field
The invention is related to high density inductively coupled plasma reactors of the type used in semiconductor wafer processing, and in particular to the inductive coil antenna which inductively couples RF plasma source power to the plasma.
2. Background Art
It is difficult to maintain a uniform ion density distribution in a plasma reactor of the type employed in etch and deposition processes in semiconductor integrated circuit fabrication. As semiconductor integrated circuit device geometries become increasingly dense and small, the effect of non-uniformities in plasma ion density in the plasma reactor becomes more significant. Much work has been directed to improving the uniformity of the radial distribution of plasma ion density near the surface of the semiconductor wafer or workpiece. For example, in an inductively coupled plasma reactor having an overhead coil antenna, it has been found that certain multi-radius dome shapes for the coil antenna and for the ceiling provide the greatest uniformity in radial distribution of plasma ion density. This is described in U.S. patent application Ser. No. 08/778,051, filed Jan. 2, 1997, by Gerald Yin et al., entitled RF PLASMA REACTOR WITH HYBRID CONDUCTOR AND MULTI-RADIUS DOME CEILING and assigned to the assignee of the present application. Such coil geometries govern the vertical displacement of the coil from the plasma or workpiece and are critical in achieving a more uniform plasma ion density distribution over the wafer or workpiece surface. As circuit device geometries become smaller, variations in the vertical height from the optimal coil geometry have greater effect on plasma processing results. In other words, the process is very sensitive to deviations in height of the coil antenna from the ideal geometry. One consequence of this is that processing performance is inconsistent and not reproducible between reactors of the identical design. It is felt that a principal cause is the variations in coil antenna geometry between different plasma reactors.
A related problem is that in those reactors having a multi-radius dome ceiling of the ideal shape for enhancing plasma uniformity, the coil antenna must conform closely to the ceiling, and must be located close to the ceiling along the entire length of the coil. One result of this is that the coil induces sputtering of the ceiling near the high voltage end of the coil due to the coil""s close proximity to the ceiling. Such sputtering produces contaminants that degrade the performance of the plasma reactor. Because of the concerns described above about maintaining a uniform plasma ion density across the wafer or workpiece surface, the proximity of the coil to the ceiling cannot be modified to solve this problem.
Another problem with inductively coupled reactors is that the reactance of the coil antenna is primarily inductive, while plasma ignition is best facilitated by capacitive coupling. As a result, plasma ignition using a coil antenna with certain gases such as SF6 that are relatively difficult to ignite requires a large amount of power, sometimes exceeding the power level of the desired process recipe.
The invention is embodied in a plasma reactor including a chamber enclosure capable of containing process gases, a workpiece support for supporting a semiconductor workpiece within the chamber enclosure, a coil antenna adjacent an exterior surface of the chamber enclosure, the coil antenna having plural windings of a conductor, the windings having a flattened cross-sectional shape defining a major flattened surface thereof generally facing an interior of the chamber enclosure, and an RF power supply connected to the coil antenna.